Subscriber station for a serial bus system and method for communicating in a serial bus system

ABSTRACT

A subscriber station for a serial bus system. The subscriber station has a communication control device for controlling a communication of the subscriber station with at least one other subscriber station, a transmitting/receiving device for transmitting a transmission signal generated by the communication control device, and a scheduling unit for scheduling a temporal access of the subscriber station to the bus in at least one time slot of a cycle of temporally consecutive time slots. At least one time slot is provided in a cycle for each subscriber station of the bus for transmitting its transmission signal and the cycle repeats cyclically. The scheduling unit determines, together with the other subscriber stations of the bus in the operation of the bus system using a priority of the transmission signal, which time slot of the cycle the transmitting/receiving device may use for transmitting the transmission signal on the bus.

FIELD

The present invention relates to a subscriber station for a serial bussystem and to a method for communicating in a serial bus system, whichallow for communication in real-time critical applications.

BACKGROUND INFORMATION

For communication between sensors and control units, for example invehicles, a bus system is used ever more frequently, instead of apoint-to-point connection, for reasons of costs, data being transmittedin the bus system as messages by CAN FD in the ISO 11898-1:2015 standardas the CAN protocol specification. The messages are transmitted betweenthe subscriber stations of the bus system such as sensors, controlunits, transmitters, etc. Currently, CAN FD is used in the introductoryphase in the first step usually at a data bit rate of 2 MBit/s in thetransmission of bits of the data field and at an arbitration bit rate of500 kbit/s in the transmission of bits of the arbitration field in thevehicle.

To prevent collisions of messages of different subscriber stations onthe bus, CAN uses the CSMA/CR method (CR=collision resolution).Collisions are thereby resolved using a so-called arbitration at thebeginning of a message or a frame. In the arbitration, an identifier(ID) is evaluated to determine which message may be transmitted next. Inthe process, the message or frame prevails whose identifier has thehighest priority. This corresponds to strict priority scheduling. Thissuffices for many applications in the autonomous vehicle.

The arbitration has the effect, however, that current CAN-based bussystems cannot be used for cases of application that require a 100%deterministic bus access, that is, a guarantee that a subscriber stationof the bus system is definitely able to transmit a message or a frame ata certain time.

SUMMARY

It is an objective of the present invention to provide a subscriberstation for a serial bus system and a method for communicating in aserial bus system, which resolve the aforementioned problems. Inparticular, a subscriber station for a serial bus system and a methodfor communication are to be provided, which are usable in acommunication for real-time critical applications and in which inparticular a 100% deterministic bus access is possible.

The objective may be attained by a subscriber station for a serial bussystem in accordance with the present invention. In accordance with anexample embodiment of the present invention, the subscriber station hasa communication control device for controlling a communication of thesubscriber station with at least one other subscriber station of the bussystem, a transmitting/receiving device for transmitting a transmissionsignal generated by the communication control device in a frame on a busof the bus system, and a scheduling unit for scheduling a temporalaccess of the subscriber station to the bus in at least one time slot ofa cycle of temporally consecutive time slots, at least one time slotbeing provided in a cycle for each subscriber station of the bus fortransmitting its transmission signal and the cycle repeating cyclically,and the scheduling unit being designed to determine, together with theother subscriber stations of the bus in the operation of the bus systemusing a priority of the transmission signal, which time slot of thecycle the transmitting/receiving device may use for transmitting theframe for the transmission signal on the bus.

In a bus system, to which the subscriber stations described above areconnected, it is ensured that every subscriber station isdeterministically able to access the bus. Consequently, each of thesubscriber stations receives a guaranteed minimum communicationbandwidth on the bus at least for the time, during which the bus is tobe accessed deterministically. This makes it possible to implementreal-time critical applications using even a CAN-based bus system.

The subscriber station may be used with any communication protocol thatoperates in accordance with the CSMA/CR method, for example with anyCAN-based communication protocol, in particular, however, with CAN XL, aCAN FD successor.

In the subscriber station according to an example embodiment of thepresent invention, it is also very advantageous that it is possible toactivate the function of the deterministic scheduling for thedeterministic bus access, as needed, even in continuous operation. Thesubscriber station is thus designed to carry out two differentcommunication methods, namely, to carry out either a CSMA/CR method,which implements “strict priority scheduling”, or a CSMA/CR method withadditional deterministic scheduling. This is a great advantage comparedto bus systems, in which respectively only one of the mentionedcommunication methods is possible, such as for example 10BASE-T1S,Flexray, etc. A further very great advantage is that the approachdescribed here is considerably simpler in terms of configuration and usethan previous methods.

The subscriber station in accordance with an example embodiment of thepresent invention is designed in such a way that it itself organizes,together with the other subscriber stations on the bus, which subscriberstation may transmit the next message. The configuration of thesubscriber station and of the associated bus system is thusself-organizing. This makes the configuration very simple to accomplish.As a result, there is hardly any additional personnel training requiredfor configuring the bus system and the subscriber station.

An additional advantage of the subscriber station is the fact that no“single point of failure” is possible, as is the case in the master noderequired for 10BASE-T1S. That is to say, when a subscriber station failsdue to a defect and therefore no longer transmits anything, this has nonegative effects on the communication on the bus.

Depending on the implemented variant of the deterministic scheduling,the subscriber station may provide either the same maximum delays (worstcase delays) as PLCA of 10BASE-T1S or maximum delays (worst case delays)that are shorter by approximately 50% than PLCA of 10BASE-T1S.

Additionally, it is possible to extend the subscriber station, dependingon the application or desire, with many additional functions, since thearbitration is always available. For transmission, it is possible forexample to make use of an unused time slot of another subscriber stationof the bus system. It is also possible to implement a weighted cyclicaltransmission (weighted round robin), in which some subscriber stationsare allowed to transmit more messages per cycle or round than othersubscriber stations. Another option is for a subscriber station of thebus system to be able to transmit in its time slot during a cycle alsomultiple shorter messages instead of a message of maximum length. Thisallows for fairness regarding the bandwidth available on the bus,instead of merely with regard to the number of messages transmitted ortransmittable by each subscriber station.

As a consequence, the subscriber station can be used to implement atransmission and reception of messages with great flexibility withregard to bus access, and thereby a very great range of quality ofservice requirements and thus applications may be realized.

Advantageous further developments of the subscriber station aredisclosed here.

According to one variant of the present invention, the scheduling unitis designed to determine, together with the other subscriber stations ofthe bus, a transmission order on the bus for the subscriber stations ofthe bus in an at least temporarily dynamic manner and/or in an at leasttemporarily static manner.

According to a special variant of an embodiment of the presentinvention, the scheduling unit has a counting module, which is designedto increment its count value with every frame received from the bus andto increment it with every transmission opportunity for a time slot thatelapsed unused, the counting module being designed to reset its countvalue to a starting value when the count value equals the number of timeslots that are provided in the cycle.

In this case, the counting module may be designed to increment its countvalue with every frame received from the bus following the reception ofa bit, which signals the beginning of a frame, even if the frame islater aborted by the subscriber station transmitting the frame due to anerror. In this case, the scheduling unit may be designed to approve atemporal access of the subscriber station to the bus for the next timeslot of the cycle if the count value of the counting module is equal tothe number of time slots that are provided in the cycle, the countingmodule being designed to set its count value to 1 if thetransmitting/receiving device has transmitted a transmission signalgenerated by the communication control device in a frame or at least onebit of the frame on the bus or lets the transmission opportunity elapseunused.

According to one exemplary embodiment of the present invention, thescheduling unit is designed to approve a temporal access of thesubscriber station to the bus in a time slot of the cycle if thesubscriber station or another subscriber station of the bus lets itstransmission opportunity elapse unused.

It is possible for the communication control device to be designed topartition the frame, at least in a switch-on phase of the bus, into afirst communication phase and a second communication phase, it beingnegotiated in the first communication phase, which of the subscriberstations of the bus obtains an at least temporarily exclusive,collision-free access to the bus in the subsequent second communicationphase.

It is possible that the number of time slots per cycle is greater thanthe number of time slots assigned to the subscriber stations of the busper cycle, it being negotiated in the first communication phase in atime slot that is not assigned to a subscriber station of the bus, whichof the subscriber stations of the bus gains at least temporarilyexclusive, collision-free access to the bus in the subsequent secondcommunication phase.

Optionally, the minimum duration of a time slot may be selected as a bittime of a bit of the first communication phase.

Optionally, the scheduling unit may be designed to transmit, in a timeslot assigned to the subscriber station, a frame with a priority that ishigher than a priority of a frame, which the scheduling unit is designedto transmit in a time slot that is assigned to another subscriberstation of the bus.

Alternatively, it is possible that the minimum duration of a time slotis two bit times of a bit of the first communication phase, thescheduling unit being designed to approve a temporal access of thesubscriber station to the bus in the second bit of a time slot of thecycle, if the subscriber station or another subscriber station of thebus in the first bit of the time slot lets its transmission opportunityelapse unused.

The communication control device is possibly designed to include in thetransmission signal a subscriber station number, which on the bus isexclusively assigned to the subscriber station, the scheduling unitbeing prepared to approve a temporal access of the subscriber station tothe bus in the time slot assigned to the subscriber station, if thescheduling unit is able to evaluate a subscriber station number in aframe received from the bus.

In one development of the present invention, the number of time slotsthat are assigned to the subscriber station per cycle may be at leasttemporarily unequal to a number of time slots that are assigned toanother subscriber station of the bus per cycle.

In another development of the present invention, the subscriber stationmay be designed to transmit more than one frame per time slot on thebus. Additionally or alternatively, the number of frames that thesubscriber station is allowed to transmit per time slot on the bus maybe at least temporarily unequal to a number of frames that anothersubscriber station of the bus is allowed to transmit per time slot onthe bus.

Optionally, the communication control device is designed to transmit atthe beginning of a recessive bit in the time slot assigned to thesubscriber station a dominant pulse that is shorter than the bit time ofthe recessive bit, if the communication control device lets itstransmission opportunity elapse unused.

According to another option of the present invention, the subscriberstation is designed in such a way that the scheduling unit may beswitched on or off depending on the time requirements of thecommunication on the bus, or that an operating mode of the schedulingunit is changeable by configuration of predetermined parameters in thecontinuous operation of the bus system, the operating mode of thescheduling unit setting a predetermined mode of a communication on thebus.

At least two of the subscriber stations described above may be part of abus system, which additionally comprises a bus, the at least twosubscriber stations being connected to one another via the bus in such away that they are able to communicate serially with one another.

The aforementioned objective is additionally achieved by a method forcommunicating in a serial bus system according to the present invention.In accordance with an example embodiment of the present invention, themethod is carried out using a subscriber station of the bus system,which comprises a communication control device and atransmitting/receiving device, the method comprising the steps ofcontrolling, using a communication control device, a communication ofthe subscriber station with at least one other subscriber station of thebus system, and transmitting, using a transmitting/receiving device, atransmission signal generated by the communication control device in aframe on a bus of the bus system according to the schedule of ascheduling unit, which schedules a temporal access of the subscriberstation to the bus in at least one time slot of a cycle of temporallyconsecutive time slots, at least one time slot being provided in a cyclefor each subscriber station of the bus for transmitting its transmissionsignal and the cycle repeating cyclically, and the scheduling unitdetermining, together with the other subscriber stations of the bus inthe operation of the bus system using a priority of the transmissionsignal, which time slot of the cycle the transmitting/receiving devicemay use for transmitting the frame for the transmission signal on thebus.

The method offers the same advantages as were mentioned above withreference to the subscriber station.

Additional possible implementations of the present invention alsoinclude combinations of features or specific embodiments not explicitlymentioned above or below with regard to the exemplary embodiments. Oneskilled in the art will also add individual aspects as improvements orsupplementations to the respective basic form of the present invention,in view of the disclosure herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described below in greater detail withreference to the figures and on the basis of exemplary embodiments.

FIG. 1 shows a simplified block diagram of a bus system according to afirst exemplary embodiment of the present invention.

FIG. 2 shows a diagram illustrating the structure of messages that maybe transmitted by subscriber stations of the bus system according to thefirst exemplary embodiment of the present invention.

FIG. 3 shows a timing diagram illustrating the sequence of acommunication in the bus system according to the first exemplaryembodiment of the present invention.

FIG. 4 shows a timing diagram illustrating the sequence of acommunication in the bus system according to the first exemplaryembodiment of the present invention, after a subscriber station waswoken up again.

FIG. 5 shows a timing diagram illustrating the sequence of acommunication in a bus system according to a second exemplary embodimentof the present invention.

FIG. 6 shows a timing diagram illustrating the sequence of acommunication in a bus system according to a third exemplary embodimentof the present invention.

FIG. 7 shows a timing diagram illustrating the sequence of acommunication in a bus system according to a fourth exemplary embodimentof the present invention.

FIG. 8 shows a time characteristic of a differential bus signal inunused time slots in a bus system according to a fifth exemplaryembodiment of the present invention.

Unless indicated otherwise, identical or functionally equivalentelements are provided with the same reference characters in the figures.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows as an example a bus system 100, which is fundamentallydeveloped in particular as a CAN bus system and/or a CAN FD bus systemand/or a CAN FD successor bus system, which is here called a CAN XL bussystem, and/or variants of the same, as described below. Bus system 100may be used in a vehicle, in particular a motor vehicle, in an aircraft,etc., or in a hospital, etc.

In FIG. 1, bus system 100 has a plurality of subscriber stations 10, 20,30, which are respectively connected to a bus 40 having a first bus wire41 and a second bus wire 42. Bus wires 41, 42 may also be called CAN Hand CAN L or CAN-XL H and CAN-XL L and are used for electrical signaltransmission after coupling in the dominant levels or generatingrecessive levels for a signal in the transmission state. Messages 45, 46are serially transmittable via bus 40 in the form of signals between theindividual subscriber stations 10, 20, 30. If an error occurs in thecommunication on bus 40, as illustrated by the jagged black arrow inFIG. 1, an error frame (error flag) 47 may be optionally transmitted.Subscriber stations 10, 20, 30 are for example control units, sensors,display devices, etc. of a motor vehicle.

As shown in FIG. 1, subscriber station 10 has a communication controldevice 11, a transmitting/receiving device 12 and a scheduling unit 15including a counting module 151. Subscriber station 20 has acommunication control device 21, a transmitting/receiving device 22 anda scheduling unit 25 including a counting module 251. Subscriber station30 has a communication control device 31, a transmitting/receivingdevice 32 and a scheduling unit 35 including a counting module 351.Transmitting/receiving devices 12, 22, 32 of subscriber stations 10, 20,30 are each connected directly to bus 40, even if this is notillustrated in FIG. 1.

Communication control devices 11, 21, 31 are respectively used forcontrolling a communication of the respective subscriber station 10, 20,30 via bus 40 with at least one other subscriber station of thesubscriber stations 10, 20, 30 that are connected to bus 40.

Communication control devices 11, 31 generate and read first messages45, which are CAN messages for example that are constructed on the basisof a CAN XL format, which is described in more detail with reference toFIG. 2. Communication control devices 11, 31 may be additionallydesigned to provide, as needed, a Can XL message 45 or a CAN FD message46 for transmitting/receiving devices 12, 32 or to receive such from thelatter. Communication control devices 11, 31 thus generate and read afirst message 45 or a second message 46, the first and second messages45, 46 being differentiated by their data transmission standard, namely,in this case CAN XL or CAN FD.

Communication control device 21 may be developed like a conventional CANcontroller in accordance with ISO 11898-1:2015, in particular like a CANFD-tolerant classical CAN controller or a CAN FD controller.Communication control device 21 generates and reads second messages 46,for example classical CAN messages or CAN FD messages 46. The CAN FDmessages 46 may comprise a number from 0 to 64 data bytes, whichadditionally are transmitted at a markedly faster data rate than in thecase of a classical CAN message. In particular, except for unit 25,communication control device 21 is developed like a conventional CAN orCAN FD controller.

Transmitting/receiving device 22 may be developed like a conventionalCAN transceiver in accordance with ISO 11898-2:2016 or CAN FDtransceiver. Transmitting/receiving devices 12, 32 may be designed toprovide, as needed, messages 45 or bits of a frame in accordance withthe CAN XL format or messages 46 or bits of a frame in accordance withthe current CAN FD format for the associated communication controldevice 11, 31 or to receive such from the latter.

Using the two subscriber stations 10, 30, it is possible to generate andthen transmit messages 45 in the CAN XL format and to receive suchmessages 45.

FIG. 2 shows in its upper section for message 46 a CAN FD frame 460, asit is transmitted serially on bus 40 by transmitting/receiving device 12or transmitting/receiving device 22 or transmitting/receiving device 32over time t. The lower section of FIG. 2 shows for message 45 a specialexample of a CAN XL frame 450, as it may be transmitted serially on bus40 by transmitting/receiving device 22 or 32 over time t. Alternatively,the upper section of FIG. 2 may be interpreted as a classical CAN frameand the lower section of FIG. 2 may be interpreted as a CAN FD frame orCAN XL frame.

According to FIG. 2, for the CAN communication on bus 40, frames 450,460 are partitioned into different communication phases 451, 452, 453,namely, an arbitration phase 451, a data phase 452, and an end of framephase 453. In the arbitration phase 451 at the start of frame 450, 460,the associated transmitting/receiving device 12, 22, 32 transmits anidentifier 451 x and a portion of a control field. In the data phase452, the following data are transmitted inter alia: a portion of thecontrol field, the user data of the CAN XL frame or of message 45, 46 ina data field DF and a checksum. In a frame 450, a portion of the controlfield may be an optional DataType field DT, which indicates the type ofdata that are transmitted in data field DF. For illustrative purposes,DataType field DT is shown in FIG. 2 having a greater length than itoften has in relation to the length of the data field DF. DataType fieldDT may be transmitted in the control portion of the frame 450, 460, inparticular at the start of data phase 452, or at the end of arbitrationphase 451. Data phase 452 is followed by the end of frame phase 453,which also belongs to the arbitrations phase according toISO11898-1:2015. End of frame phase 453, which may also be referred toas the arbitration phase at the end of frame 450, 460, has inter aliathe following portions: an ACK field and an end of frame (EOF)indicator. The end of frame phase 453 is not relevant in this contextand is therefore not described in more detail.

In the arbitration phase 451, the associated transmitting/receivingdevice 12, 22, 32 transmits bits of frame 450, 460 at a slower bit ratethan in the data phase 452. In the case of CAN FD, the data phase 452 istemporally markedly shorter than the data phase 452 of the classical CANframe. In particular application cases, the two bit rates of phases 451,452 may be configured to the same values, but usually the bit rate inthe data phase 452 is considerably higher than in the arbitration phase451.

For CAN XL, a frame format is defined, in which not only the bit rateswithin frame 450 or message 45 are switched over, but optionally alsothe operating mode of transmitting/receiving device 12, 32. In thearbitration phase 451, transmitting/receiving device 12, 32 works in anoperating mode (here called CAN) that is compatible to ISO 11898-2:2016.In the data phase 452 of frame 450, transmitting/receiving device 12, 32may be optionally switched into another operating mode that allows forhigher bit rates and thus a fast data transmission.

The arbitration phase 451 is used to negotiate with the aid of anidentifier (ID) 451 x bitwise between subscriber stations 10, 20, 30 todetermine which subscriber station 10, 20, 30 has the message 45, 46with the highest priority and hence receives exclusive access to bus 40of bus system 100 for transmitting at least in the subsequent data phase452. For this purpose, the conventional CSMA/CR method is applied in thearbitration phase 451, which allows for simultaneous access ofsubscriber stations 10, 20, 30 to bus 40, without the higher prioritizedmessage 45, 46 being destroyed. This makes it possible in a relativelysimple manner to add further bus subscriber stations 10, 20, 30 to bussystem 100, which is very advantageous.

In the arbitration phase 451, the associated transmitting/receivingdevice 12, 22, 32 thus uses a physical layer as in the case of CAN andCAN FD. The physical layer corresponds to the bit transmission layer orlayer 1 of the conventional OSI model (open systems interconnectionmodel).

If scheduling units 15, 25, 35 are not activated, frames 450, 460 aregenerated and the bus access of the individual subscriber stationsproceeds in an uncoordinated manner. Conflicts in the communication onbus 40 are resolved by an arbitration, as defined in ISO 11898-1:2015.

The CSMA/CR method has the consequence that there must be so-calledrecessive states on bus 40, which may be overwritten by other subscriberstations 10, 20, 30 having dominant states on bus 40. In the recessivestate, highly resistive conditions prevail at the individual subscriberstations 10, 20, 30, which in combination with the parasites of the buscircuit entail longer time constants. This results in a limitation ofthe maximum bit rate of today's CAN FD physical layer to currentlyapproximately 2 megabits per second in real vehicle usage.

A transmitter of message 45, 46 starts a transmission of bits of dataphase 452 on bus 40 only when the corresponding subscriber station 10,20, 30 as the transmitter won the arbitration and subscriber station 10,20, 30 as transmitter thus has exclusive access to bus 40 of bus system100 for transmitting.

Quite generally, in the bus system using CAN XL, the following deviatingproperties may be implemented in comparison to CAN or CAN FD:

-   -   a) take-over and, if indicated, adaptation of proven properties        that are responsible for the robustness and user-friendliness of        CAN and CAN FD, in particular the frame structure with        identifier 451 x and arbitration in accordance with the CSMA/CR        method,    -   b) increase of the net data transmission rate, in particular to        approximately 10 megabits per second,    -   c) increase of the size of the user data per frame, in        particular to approximately 4 kbyte.

FIG. 3 shows a case for a communication on bus 40, in which schedulingunits 15, 25, 35 are activated. In this case, a deterministic bus accessof each of the subscriber stations 10, 20, 30 is possible. For thispurpose, a transmission cycle or cycle C is used in the communication onbus 40, in which in the simplest case one time slot S is available foreach of subscriber stations 10, 20, 30. The number SN of time slots Sthus corresponds to the number N of subscriber stations 10, 20, 30 onbus 40. The bandwidth on bus 40 is divided by the number N of subscriberstations 10, 20, 30.

In the example of FIG. 3, there are four time slots S, namely, timeslots S1, S2, S3, S4, so that four arbitrary subscriber stations ofsubscriber stations 10, 20, 30 are present. The four arbitrarysubscriber stations of subscriber stations 10, 20, 30 are thus referredto below as subscriber stations 1, 2, 3, 4.

Quite generally, SN time slots S may be provided, SN being an arbitrarynatural number. The number of time slots S is constant, that is, it isthe same in very cycle C.

Scheduling units 15, 25, 35 effect a generation of frames 450, 460according to the following rules.

Each frame 450, 460 again starts with an arbitration phase 451. Theidentifier 451 x may be chosen arbitrarily for each frame 450, 460, aslong as the rule, customary in CAN, is observed, that each identifier451 x is used exclusively only by one of subscriber stations 1, 2, 3, 4.

The minimum time slot duration T_S_mn is at least one arbitration bittime, that is, as long as the bit time of a bit in arbitration phase451. The maximum time slot duration T_S_mx is equal to the maximumlength of a frame 460 or 450. The minimum cycle duration T_C_mn is thusequal to the sum of all minimum time slot durations T_S_mn. Moreover,the maximum cycle duration T_C_mx is equal to the sum of all maximumtime slot durations T_S_mx.

Each subscriber station 1, 2, 3, 4 may transmit on bus 40 only in thetime slot S1, S2, S3, S4 that is provided for the subscriber stations 1,2, 3, 4. In the present exemplary embodiment, each subscriber station 1,2, 3, 4 may transmit a single frame 450, 460 on bus 40 in the time slotS1, S2, S3, S4 provided for it.

In its time slot S1, S2, S3, S4, each of the subscriber stations 1, 2,3, 4 respectively has one transmit opportunity TO. Thus, the subscriberstation 1, 2, 3, 4 may transmit a frame 450, 460 in the time slot S1,S2, S3, S4 assigned to it, or it may let the opportunity fortransmission elapse. Following a minimum time slot duration T_S_mn, thetransmit opportunity TO has elapsed.

In the example of FIG. 3, the minimum cycle duration T_C_mn is fourarbitration bit times, because 4 time slots S1, S2, S3, S4 are availablefor the four subscriber stations 1, 2, 3, 4. The minimum time slotduration T_S_mn is assumed as 1 arbitration bit time.

The following basic assumptions apply to the present exemplaryembodiment. In the first cycle C, that is, after the bus system 100 isswitched on, time slots S1, S2, S3, S4 are assigned to the individualsubscriber stations 1, 2, 3, 4. The assignment occurs dynamically viathe frame Ids or identifiers 451 x, which subscriber stations 1, 2, 3, 4use for transmitting their first frame 450, 460. The transmission orderis determined by the arbitration, which corresponds to the assignment oftime slots S1, S2, S3, S4 to the subscriber stations 1, 2, 3, 4.

If subscriber stations 1, 2, 3, 4 are not switched on simultaneously,the assignment of time slots S1, S2, S3, S4 is completed only when thelast subscriber station 1, 2, 3, 4 has been switched on and when everysubscriber station 1, 2, 3, 4 has transmitted a frame 450, 460. Theassignment of time slots S1, S2, S3, S4 to subscriber stations 1, 2, 3,4 is maintained in the subsequent cycles C. That is, there is no newarbitration and the transmission order remains unchanged. If one orseveral of the subscriber stations 1, 2, 3, 4 go to sleep and wake upagain, a reintegration is performed, which requires again anarbitration. This will be described in more detail later with referenceto FIG. 4.

FIG. 3 shows a start cycle C_SU, in which in a time duration T_C_SU, theassignment of time slots S1, S2, S3, S4 to subscriber stations 1, 2, 3,4 is determined. For the sake of simplicity, it is assumed in theexample of FIG. 3 that all subscriber stations 1, 2, 3, 4 want totransmit right at the start of a frame 450, 460.

In time slot S1, all subscriber stations 1, 2, 3, 4 start a framesimultaneously and participate in the bus arbitration, as illustrated byA1234 in FIG. 3. Subscriber station 4 wins the arbitration A andtransmits its frame. This is illustrated in FIG. 3 by TX4, TX4 standingfor the transmission signal TX of subscriber station 4 and the framebeing based on transmission signal TX4. Time slot S1 is thereby assignedto subscriber station 4.

In time slot S2, subscriber stations 1, 2, 3 start a framesimultaneously and participate in the bus arbitration, as illustrated byA123 in FIG. 3. Subscriber station 2 wins the arbitration and transmitsits frame. This is illustrated by TX2 in FIG. 3. Time slot S2 is therebyassigned to subscriber station 2.

For time slot S3, an arbitration is performed between subscriberstations 1, 3, as described above. Finally, in the example of FIG. 3,time slot S3 is assigned to subscriber station 1.

Time slot S4 is assigned to subscriber station 3. An arbitration is nolonger performed because the other subscriber stations 1, 2, 4 do nottransmit anything in time slot S4.

Hence, the TX transmission order is specified by the arbitrationhereinafter as TX_4_2_1_3.

Subsequently, a normal operation of bus system 100 can begin. A possiblecycle C in the operation could be a cycle C_B, which is illustrated inFIG. 3 in the right section of the figure. In the cycle C_B, the TXtransmission order continues to be specified without change asTX_4_2_1_3. Subscriber stations 3, 4 transmit in cycle C_B. Subscriberstations 1, 2, on the other hand, do not transmit. Subscriber stations3, 4 thus make use of their transmit opportunity TO. Transmit stations1, 2, however, let their transmit opportunity TO elapse. This yields anaverage cycle duration T_C_B. The maximum cycle duration T_C_mx resultswhen in every time slot S1 through S4 the associated subscriber station1, 2, 3, 4 transmits a frame 450, 460 of the maximum length. The number1 in time slot S3 indicates that this time slot is assigned tosubscriber station 1. The number 2 in time slot S2 indicates that thistime slot is assigned to subscriber station 2. This similarly applies totime slots S1 and S4 and subscriber stations 3, 4.

For implementing the above-described scheduling for a transmission offrames 450, 460 on bus 40, each of the scheduling units 15, 25, 35 isconstructed as described below.

Each subscriber station 1, 2, 3, 4 on bus 40 knows the number SN of timeslots S1, S2, S3, S4 per cycle C. In the simplest case, the number ofsubscriber stations 1, 2, 3, 4 on bus 40 corresponds to the number oftime slots S1, S2, S3, S4 per cycle C, so that this yields a 1-1association of time slots and subscriber stations. The number SN of timeslots S1, S2, S3, S4 per cycle C is stored in scheduling units 15, 25,35.

The normal operation for a subscriber station 1, 2, 3, 4 starts as soonas the subscriber station 1, 2, 3, 4 has transmitted its first frame450, 460. After the frame 450, 460 has been transmitted, counting module151, 251, 351 of scheduling units 15, 25, 35 sets its count value Scntto 1. From now on, counting module 151, 251, 351, using at least onecounter, counts the number of received frames 450, 460 on bus 40 and thenumber of elapsed transmit opportunities TO. For this purpose, countingmodule 151, 251, 351 counts per received frame 450, 460 and elapsedtransmit opportunity TO, so that the count value Scnt of the at leastone counter is respectively incremented by 1 and thus Scnt:=Scnt+1.

As soon as the start of frame bit of a frame 450, 460 is transmitted onbus 40, counting module 151, 251, 351 counts the frame 450, 460 as atransmitted frame 450, 460 on bus 40. This also holds true if thetransmission is aborted by the transmitter, for example by an errorframe 47 due to an error.

If in counting module 151, 251, 351 for the count value Scnt==SN, thenext time slot is its own time slot. In the case of the count valueScnt==SN, the subscriber station 1, 2, 3, 4 thus has its own transmitopportunity TO in the next time slot. Now the respective subscriberstation 1, 2, 3, 4 is able to transmit a frame 450, 460 or let thetransmit opportunity TO elapse.

Even when subscriber station 1, 2, 3, 4 lets its transmit opportunity TOelapse, the associated counting module 151, 251, 351 of the subscriberstation 1, 2, 3, 4 nevertheless sets its count value Scnt:=1.

Moreover, the counting module 151, 251, 351 of a subscriber station 1,2, 3, 4 sets its count value to Scnt:=1 only when the subscriber station1, 2, 3, 4 was able to transmit its frame or has let its transmitopportunity TO elapse. This is necessary when the transmission order onbus 40 is assigned anew and another arbitration ensues as a result. Thismay be the case if one of the subscriber stations 1, 2, 3, 4 was wokenup again and it then wants to participate again in the buscommunication. This is described in more detail below with reference toFIG. 4.

After having been switched on or woken up, the subscriber stations 1, 2,3, 4 do not know which of the time slots S1, S2, S3, S4 per cycle C isassigned to the respective subscriber station 1, 2, 3, 4.

FIG. 4 shows the case in which subscriber station 3 was initially asleepand was then woken up again. At time t1, for example, subscriber station3 in FIG. 4 has woken up again and is ready to transmit. Up until thispoint in time and thus also up until the point in time before subscriberstation 3 was put to sleep, the transmission order was determined asTX_1_2_3_4, as shown on the left side in FIG. 4 by the numbers in thetime slots. Since subscriber station 3 has lost its synchronization withthe time slot number, subscriber station 3 attempts to transmit forexample a frame 450 at the next opportunity, that is, when bus 40 isfree. Coincidentally, in the example of FIG. 4, subscriber station 3attempts to transmit the first frame 450 precisely at the time t1, whenits actual transmit opportunity TO in time slot S3 has elapsed.

The frame 450 of subscriber station 3 results in an arbitration on bus40 if the current time slot, time slot S4 in the example shown, isalready assigned to another of the subscriber stations 1, 2, 4 and thissubscriber station 1, 2, 4 also starts a frame 450, 460 in the currenttime slot. In the example of FIG. 4, an arbitration therefore occursbetween the frames 450, 460 of subscriber stations 3, 4, as illustratedby A34 in FIG. 4. The newly started subscriber station 3 thus attemptsliterally to insert itself into the communication.

If the frame 450 of subscriber station 3 loses the arbitration,subscriber station 3 repeats the transmission of frame 450 until thesubscriber station 3 has won the arbitration and is able to transmit itsframe 450 or, after winning the arbitration, aborts due to an error.Subsequently, the counting module 151, 251, 351 of subscriber station 3sets its count value Scnt:=1 and thus begins again at the start of thecount.

In the example of FIG. 4, frame 450 of subscriber station 3 won thearbitration in time slot S4. Subscriber station 3 thus took over timeslot S4 from subscriber station 4. In other words, subscriber station 4wanted to transmit one of frames 450, 460, but lost the arbitration onbus 40. Subscriber station 4 thus had its time slot S4 just taken awayby another one of the subscriber stations. In the example of FIG. 4,this is the subscriber station 3, which was started anew and took awaythe time slot from subscriber station 4.

For this reason, the frame of subscriber station 4 must now arbitratewith the frame of subscriber station 1 in the subsequent time slot S1.Since the frame of subscriber station 1 wins the arbitration here, theframe of subscriber station 4 must arbitrate in time slot S2 with theframe of subscriber station 2. Ultimately, subscriber station 4 is ableto transmit its transmission signal TX4 or the associated frame 450, 460not until time slot S3, which previously was assigned to subscriberstation 3. The transmission of transmission signal TX4 is thus delayedby the time duration T_D_TX4.

Accordingly, in the example of FIG. 4, or depending on the result of thearbitration, the assignment of time slots S1, S2, S3, S4 is distributedanew. Thus, in the example of FIG. 4, the previous transmission order isno longer specified as

TX_1_2_3_4, but is changed to TX_1_2_4_3. In this example, subscriberstations 3, 4 have swapped their assigned time slots S3, S4, as shown onthe right in FIG. 4 by the numbers in the time slots.

This new transmission or time slot assignment has the result that thedelay in time in the worst case (worst case delay) becomes longer forthe bus access of subscriber station 4 than the maximum cycle durationT_C_mx. In the worst case, the delay is longer by SN−1 maximum framelengths than the maximum cycle duration T_C_mx, i.e., here longer bySN−1=4−1=3. The worst case delay for the bus access of a subscriberstation 1, 2, 3, 4 is thus S+S−1=2S−1 maximum frame lengths. This iscomparable to the PLCA of 10BASE-T1S, which has approximately the samevalue.

One advantage of this exemplary embodiment is that the communication onbus 40 through the arbitration is self-organizing. Hence, no master nodeis required to organize the communication. Because there exists nomaster node, there is also no “single point of failure”, which couldcause the communication on bus 40 to come to a halt.

A further advantage of this exemplary embodiment is that in theconfiguration of bus system 100, only a single parameter needs to beentered, namely, the parameter SN, which is equal to the number N of thesubscriber stations on bus 40. The configuration thus requires verylittle expenditure.

FIG. 5 shows a time sequence in bus system 100 for illustrating acommunication according to a second exemplary embodiment. For thispurpose, FIG. 5 shows a cycle when switching on a subscriber station 1,2, 3, 4, that is, a switch-on cycle C_E, and a subsequent normaloperation with normal cycles C_N.

In contrast to the time sequence of the communication according to theabove-described exemplary embodiment, in the present exemplaryembodiment it is also possible that, after being switched on or wokenup, a subscriber station 1, 2, 3, 4 is more quickly synchronized to thetime slot number than in the above-described exemplary embodiment. Thisis achieved with the aid of a subscriber station number, which istransmitted as information in frame 450, 460. It may thus be seen on bus40 or by evaluating the frame received from bus 40, which subscriberstation 1, 2, 3, 4 of bus 40 is currently transmitting. Consequently, itis known indirectly which number the current time slot has. To this end,the following procedure is followed in the present exemplary embodiment.

Every subscriber station 1, 2, 3, 4 on bus 40 or, more precisely, everyscheduling unit 151, 251, 351 knows the number SN of time slots S1, S2,S3, S4 per cycle C. The number SN may be entered as parameter SN in theconfiguration of bus system 100 in the respective scheduling unit 151,251, 351. In the simplest case, the number N of subscriber stations 1,2, 3, 4 on bus 40 corresponds to the number SN of time slots S1, S2, S3,S4 per cycle C, so that the 1-1 association is again given.

Furthermore, every subscriber station 1, 2, 3, 4 knows the subscriberstation numbers of the other subscriber stations 1, 2, 3, 4. In thesimplest case, the subscriber station number is used as frame identifier451 x (frame ID), which is transmitted at the beginning of each frame450, 460. Every subscriber station 1, 2, 3, 4 thus knows which time slotS1 through S4 belongs to which subscriber station 1, 2, 3, 4. Ideally,every subscriber station 1, 2, 3, 4 additionally transmits all itsframes 450, 460 with the same frame identifier 451 x.

If for example SN=10 time slots S1 through S10 and thus also subscriberstations 1 through 10 are present on bus 40, the frame identifier 451 xand thus the subscriber station number of subscriber station 1 may bethe 0x01, that of subscriber station 2 may be the 0x02, etc., and thatof subscriber station 10 the 0x0A. In a simple assignment of subscriberstations to time slots, subscriber station 1 transmits in time slot S1,subscriber station 2 transmits in time slot S2, etc., and subscriberstation 10 transmits in time slot S10. The assignment of subscriberstations 1 through 10 to time slots S1 through S10 thus involves verylittle expenditure. It is of course also possible, however, to specifythe transmission order, which corresponds to the assignment of time slotnumber to subscriber station number, as desired.

SN=4 time slots and N=4 subscriber stations are again assumed in thefollowing.

After being switched on, every subscriber station 1, 2, 3, 4 mustobserve the communication on bus 40 for a certain time. Here, thefollowing two cases must be distinguished, which are described below.

In the first case A1, subscriber station 1 for example receives a validframe 450, 460. Subscriber station 1 knows the current time slot numberfrom the subscriber station number in the received frame 450, 460, whichis indicated for example via the frame identifier 451 x. Subscriberstation 1 is thus synchronized to time slots S1 through S4 andtransitions into normal operation.

In the second case A2, subscriber station 1, 2, 3, 4 does not receive aframe 450, 460, because the bus is idle at the moment, as may be seen inFIG. 5 on the left from the many unused transmit opportunities TO fortime slots S1 through S4 from time t2 onward.

If SN time slots of minimal duration have elapsed, without subscriberstation 2, for example, having received a frame 450, 460 after havingbeen switched on, then none of the subscriber stations 1, 2, 3, 4 has sofar transmitted in this switch-on cycle C_E. In the example of FIG. 5,the predetermined waiting time for the newly switched-on subscriberstation 2 has a time duration T_C_mn. The time duration T_C_mncorresponds to SN=4 time slots of elapsed transmit opportunity TO.

For this reason, subscriber station 2 is simply able to transmit itsframe 450, 460 at time t2. Subscriber station 2 thus transmits mostprobably in a time slot that is assigned to another one of thesubscriber stations 1, 3, 4. This is no problem, however. For, due tothe long preceding idle phase on bus 40, which corresponds to manyelapsed transmit opportunities TO, the delay for the bus access in theworst case does not increase. If multiple subscriber stations 1, 2, 3, 4simultaneously start a transmission in a time slot S1, S2, S3, S4 and anarbitration thereby occurs on bus 40, then the frame that won thearbitration defines the time slot number anew.

According to the example of FIG. 5, a predetermined waiting time, whichcorresponds to a minimum cycle duration T_C_mn, has elapsed forsubscriber station 2. Subscriber station 2 is therefore now allowed totransmit. The time slot beginning at time t2 in switch-on cycle C_E isactually assigned to subscriber station 4. This results in the casewhere subscriber station 2 and subscriber station 4 in the example ofFIG. 5 start to transmit a frame in the time slot after time t2. This isillustrated in FIG. 5 as A24, TX2. The frames of subscriber stations 2,4 thus participate in the bus arbitration. In the example of FIG. 5,subscriber station 2 prevails, since subscriber station 2 has anidentifier 451 x of a higher priority in the frame.

Due to the transmitted frame 450, 460 after time t2, the respectivescheduling unit 151, 251, 351 of subscriber stations 1, 2, 3, 4 changesthe previous time slot number S4 for all subscriber stations 1, 2, 3, 4to the number to which the transmitted frame is entitled. The time slotafter time slot S3 or after t2 is thus numbered anew as time slot S2, asindicated in FIG. 5.

Thereupon, that is, at or beginning with time t3, all subscriberstations 1, 2, 3, 4 know the current time slot number, which is here S3.

In normal operation, that is, during cycles C_N, a subscriber station 1,2, 3, 4 is synchronized to the time slot numbers. In order to find itsown time slot S1, S2, S3, S4, each subscriber station 1, 2, 3, 4 usesits counting module 151, 251, 351, as described with reference to thepreceding exemplary embodiment. The counting module 151, 251, 351 isnecessary in particular because some subscriber stations 1, 2, 3, 4,like for example subscriber station 3 in its time slot S3, possibly lettheir transmit opportunity TO elapse because a sensor as subscriberstation 3 currently records no data or does not have to transmit so manydata. This is illustrated in FIG. 5 at the center. The right portion ofFIG. 5 by contrast shows a normal cycle C_N of maximum duration T_C_mx.With the aid of counting module 151, 251, 351, it is now neverthelessknown to every subscriber station 1, 2, 3, 4, which time slot or whichtime slot number is concerned. As a result, all subscriber stations 1,2, 3, 4 of bus system 100 remain synchronized to time slots S1, S2, S3,S4 or to the time slot numbers.

The one-time reception of a frame 450, 460 suffices to ensure that thescheduling unit 15, 25, 35 of the respective subscriber stations 1, 2,3, 4 is always ready to transmit a frame intended for the subscriberstation 1, 2, 3, 4 in a time slot S1, S2, S3, S4. After SN time slots,that is, after a minimum cycle duration T_C_mn, without frame 450, 460,scheduling unit 15, 25, 35 of the respective subscriber stations 1, 2,3, 4 is nevertheless ready to transmit. In this case, scheduling unit15, 25, 35 of the respective subscriber stations is able simply totransmit its frame 450, 460.

According to a modification of the embodiment described earlier, it ispossible to transmit the subscriber station number at least one otherposition in frame 450, 460. For example, the subscriber station numbermay be transmitted in the data phase 452, in particular in data fieldDF, for example at the beginning of data field DF, or with the data typefield (DataType field) DT, which is transmitted in a control section ofthe frame 450, 460, in particular at the beginning of data phase 452 orat the end of arbitration phase 451. The data type field indicates thetype of data that are transmitted in data field DF.

Thus, at least one of the scheduling units 15, 25, 35 of the subscriberstations 1, 2, 3, 4 is designed, by using at least one item ofinformation received from bus 40, to determine an assignment thatspecifies which time slot of the cycle C the transmitting/receivingdevice 12, 22, 32 may use for transmitting a frame for a transmissionsignal on bus 40. In the second exemplary embodiment and itsmodification, this item of information is the subscriber station number.

In the second exemplary embodiment and its modification, the subscriberstation number is thus predetermined by the system integrator or by theconfigurator of bus system 100. The order of the communication on bus 40is therefore no longer self-organizing, but rather it is predetermined,namely, via the assignment of the subscriber station number. Thesubscriber stations may then transmit only in a specified order, e.g.,in the ascending order of the subscriber station number.

In the second exemplary embodiment and its modification, it is veryadvantageous, however, that a subscriber station that is switched onagain is synchronized to time slot numbers S1, S2, S3, S4 existing inthe communication on bus 40 more quickly than in the first exemplaryembodiment. The worst case delay for the bus access of a subscriberstation 1, 2, 3, 4 is thus only SN maximum frame lengths both inoperation as well as when being switched on. The delay is thus only halfas long as in the above-described exemplary embodiment. This variant ofan embodiment is also more robust with respect to the counting of thetime slots since the time slot number may be seen on the bus. The firstand the second variant of an embodiment have in particular the followingadvantage. If, due to an error in one of the scheduling units 15, 25,35, a subscriber station transmits in the wrong time slot, then thismerely results in an arbitration and in a possible delay of thetransmission. Such an error, however, cannot result in the destructionof the two frames on the bus.

FIG. 6 shows a time sequence in bus system 100 for illustrating acommunication according to a third exemplary embodiment. For thispurpose, FIG. 6 shows in its left section a cycle having a minimum cycletime duration T_C_mn, in which all time slots S1 through S4 respectivelyhave at least two arbitration bit times. For example, time slot S3 has afirst bit time B1_S3 and a second bit time B2_S3. The same is true forthe other time slots S1, S2, S4.

A subscriber station 1, 2, 3, 4, which has its transmit opportunity TOin a time slot S1, S2, S3, S4, must start its frame in the first bit ofthe time slot S1, S2, S3, S4 assigned to it, in order to be guaranteedto be able to transmit a frame. Thus, no bit elapses in time slot S1,S2, S3, S4. If a subscriber station 1, 2, 3, 4 lets the first bit of itstime slot S1, S2, S3, S4 elapse, then the subscriber station 1, 2, 3, 4has also let its transmit opportunity TO elapse, which would haveguaranteed the subscriber station 1, 2, 3, 4 the transmission of aframe. Thus, subscriber station 1, 2, 3, 4 currently does not want totransmit a frame.

Consequently, another subscriber station 1, 2, 3, 4 is now able to usethis time slot S1, S2, S3, S4 for transmission. It is possible, however,that for example the subscriber station 1 lets its first bit time B1_S1of its time slot S1 elapse, but then actually begins to transmit a framein the second bit time B2_S1 of its time slot S1. In this example, it isthus possible for all subscriber stations 1, 2, 3, 4 to start a frame inthe second bit time B2_S1 of time slot S1. The frames may also “fight”over this free time slot S1, that is, arbitrate on bus 40. In order forthe frame having the highest priority to win the arbitration, subscriberstations 1, 2, 3, 4 may use identifiers 451 x for their frames thatbegin in the second bit of a time slot, which match the priority of theframe. The arbitration ensures that the frame of the highest priorityprevails.

This development of the minimum duration T_S_mn of time slots S1 throughS4 makes it possible for one of the subscriber stations 1, 2, 3, 4 touse the transmit opportunity TO of another subscriber station, becauseit is clear from a first bit time that elapsed unused that therespective subscriber station has not used its guaranteed transmitopportunity TO.

In the example of FIG. 6, subscriber station 4 uses the fourth time slotS4 that is assigned to it for transmitting the transmission signal TX4in a frame. At a t4 during time slot S4, subscriber station 3 is readyto transmit a frame. Since subscriber station 1 does not transmit aframe—the first bit time B1_S1 elapsing unused—starting at time t5,subscriber station 3 transmits the transmission signal TX3 as a frame inthe second bit time B2_S1 of time slot S1. The second and third timeslots S2, S3 are respectively used by the associated subscriber stations2, 3. transmission signals TX2, TX3 are thus respectively transmitted ina frame on bus 40. Since subscriber station 4 does not transmit aframe—the first bit time B1_S4 elapsing unused—subscriber stations 1, 2transmit frames on bus 40 starting at time t6. In the subsequentarbitration, subscriber station 2 prevails, so that transmission signalTX2 is transmitted as a frame on bus 40 in time slot S4.

One advantage of the present exemplary embodiment is that it is possibleto mix a deterministic allocation of the communication bandwidth with abest effort data traffic. The best effort data traffic only comes aboutif some subscriber stations 1, 2, 3, 4 do not use the bandwidth, thatis, the transmission slot S1, S2, S3, S4, allocated to them.

Another great advantage of the present exemplary embodiment is that theworst case delay for the bus access increases thereby only minimally. Itbecomes longer because the minimum time slot duration T_S_mn becomeslonger.

In principle, the minimum time slot duration T_S_mn may alternativelyhave a length of three or more arbitration bit times. Subscriberstations 1, 2, 3, 4 on bus 40 may then be divided into differentpriority classes. The lower the priority class of a subscriber station1, 2, 3, 4, the later the subscriber station 1, 2, 3, 4 may start atransmission within a time slot that remains free.

According to a modification of the present exemplary embodiment, thedevelopment having at least two bit times may be used so that subscriberstations 1, 2, 3, 4, after waking up or being switched on, arereintegrated in such a way that a transmitting subscriber station 1, 2,3, 4 is not disadvantaged in that the time slot S1, S2, S3, S4 assignedto it is taken away from the transmitting subscriber station 1, 2, 3, 4.For this purpose, preferably every subscriber station 1, 2, 3, 4 usesfor reintegration only time slots S1, S2, S3, S4 that were not used inthe first bit by the actual time slot owner. If the reintegratingsubscriber station of the subscriber stations 1, 2, 3, 4 transmits itsframe starting with the second bit of time slot S1, S2, S3, S4 and if itwins the arbitration and is able to transmit the frame successfully,that is, error-free, then the time slot is subsequently assigned to it.

FIG. 7 shows a time sequence in bus system 100 for illustrating acommunication according to a fourth exemplary embodiment.

In contrast to the preceding exemplary embodiments, in the presentexemplary embodiment, the communication bandwidth on bus 40 is notuniformly distributed between subscriber stations 1, 2, 3, 4. Instead,in the present exemplary embodiment, a so-called WRR method (weightedround robin method) is used. In the process, each subscriber station 1,2, 3, 4 obtains a further parameter W (weight), which is adjustable inthe associated scheduling unit 15, 25, 35 during the configuration. Theparameter W indicates the number of frames 450, 460, which thesubscriber station 1, 2, 3, 4 may transmit per cycle. W time slots areprovided for one subscriber station 1, 2, 3, 4. For this reason, theparameter SN set in all subscriber stations 1, 2, 3, 4 now no longercorresponds to the number of subscriber stations 1, 2, 3, 4, but ratherto the sum of all values of parameter W.

FIG. 7 shows an example with four subscriber stations 1, 2, 3, 4, inwhich the parameter W=2 is set for subscriber station 1. For the othersubscriber stations 2, 3, 4, however, a parameter W=1 is respectivelyset. The number SN of time slots thus computes as SN=2+1+1+1=5.Consequently, a cycle C has a number of 5 time slots, namely, S1, S2,S3, S4, S5. In this case, subscriber station 1 is able to transmit twoframes 450, 460 per round, namely, in time slots S1 and S2. The othersubscriber stations 2, 3, 4 are each able to transmit only one frame450, 460 per cycle C.

For explaining a fifth exemplary embodiment, FIG. 8 shows a differentialvoltage V-diff, which is computed from the difference of bus signals CANH and CAN L on bus 40.

According to FIG. 8, a subscriber station 1, 2, 3, which is not usingits transmit opportunity TO, transmits a short dominant pulse at thebeginning of bits B1-S1, B1_S2, B1_S3, etc., in its time slot S1, S2,S3. In the example of FIG. 8, only one bit, namely, the bit B1, istransmitted in each time slot S1, S2, S3, since the minimum time slotduration T_S_mn is assumed here to be an arbitration bit time. The bitsare scanned by the receiving subscriber stations 1, 2, 3 at a scanningtime t_A. If the minimum duration T_S_mn of a time slot corresponds tomultiple arbitration bit times, then the subscriber station, to whichthe time slot is assigned, is able to transmit a dominant pulse P at thebeginning of each bit if the subscriber station lets its transmitopportunity TO elapse.

In this case, the associated subscriber station 1, 2, 3 transmits adominant pulse P of e.g. 200 ns in the recessive bits of its time slotS1, S2, S3. All CAN subscriber stations 1, 2, 3 synchronize to therecessive-dominant edge S_F of the dominant pulse P. Since thearbitration bit time is limited to 1000 ns or 1 Mbit/s in accordancewith the aforementioned specification, a corresponding pulse P of 200 nsis not limiting. All subscriber stations 1, 2, 3 scan the actuallyrecessive bit B1 as recessive in spite of the dominant pulse P, becauseat scanning time t_A, the pulse P has already ended long ago.

This makes it possible that subscriber stations 1, 2, 3 do not lose thesynchronization to time slots S1, S2, S3, even if long idle phases occurin the communication on bus 40, in which all subscriber stations 1, 2, 3do not use their transmit opportunities TO. By way of the dominantpulses P, subscriber stations 1, 2, 3 may be kept synchronized.

Bus system 100 is thus able to handle clock pulses that have relativelyhigh tolerances. In this case, subscriber stations 1, 2, 3 synchronizeto recessive-dominant edges on bus 40.

According to a sixth exemplary embodiment, it is possible that one ormultiple subscriber stations 1, 2, 3, 4 of the preceding exemplaryembodiments transmit multiple frames per time slot S1, S2, S3, S4. Thisdevelopment is advantageous for example in order to distribute thecommunication bandwidth on bus 40 more fairly, when for examplesubscriber station 1 typically transmits short frames 460 and subscriberstation 2 typically transmits long frames 450. If each subscriberstation 1, 2 is always only able to transmit one frame in the time slotS1, S2 assigned to it, then subscriber station 1 has less bandwidth thansubscriber station 2 in the method according to the first and secondexemplary embodiments.

In order to achieve greater fairness with regard to the communicationbandwidth than in the first and second exemplary embodiments, subscriberstation 1 is for example permitted to transmit multiple short frames 450during its time slot S1. In order to remain fair, frames 450 in sumtotal should not be longer than a frame 450 of maximum length.

The fact that an additional frame 460 is transmitted by subscriberstation 1 in this transmit opportunity TO may be signaled in thefollowing ways.

For example, an identifier 451 x may be used for this purpose. In thiscase, the less significant 4 bits of the identifier (ID) 451 x are usedto identify subscriber station 1. The next 7 bits of identifier (ID) 451x are used to communicate that additional frames 460 will follow. 0x14,for example, may signal that subscriber station 4 transmits a frame, the1 signaling that the frame is followed by additional frames. 0x04 couldthen signal that subscriber station 4 transmits a frame, the 0 signalingthat the frame is not followed by additional frames.

According to another possibility for signaling that subscriber station 1is transmitting an additional frame 450 in this transmit opportunity TO,a field DataType (DT) may be used in the header of frame 450. DT=0x30,for example, may signal that no additional frame of the same subscriberstation follows. DT=0x31 could therefore signal that one additionalframe of the same subscriber station follows.

Alternatively, according to yet another possibility for signaling, it ispossible to use a dedicated bit in the header of frame 460.

Alternatively, according to yet another possibility for signaling, it ispossible to use a bit or byte at the beginning of the data field DF,shown in FIG. 2, in the data phase 452.

The advantage is that more fairness may be established with regard tothe distribution of the communication bandwidth between subscriberstations 1, 2, 3, 4 in the case in which subscriber stations 1, 2, 3, 4transmit frames 450, 460 of different lengths.

According to a seventh exemplary embodiment, it is possible that thenumber SN of the time slots is set in the scheduling units 15, 25, 35 tobe greater than the number N of subscriber stations. In this case, atleast one time slot remains, which belongs to none of the subscriberstations and thus to no one on bus 40. If this is combined with thethird exemplary embodiment, where it is explained how the othersubscriber stations are able to use unused time slots, then the surplustime slots may be used by any subscriber station.

Since in principle all subscriber stations may transmit simultaneouslyin the surplus time slots and thus arbitrate over bus 40, thiscorresponds to strict priority scheduling.

The bus has N=4 subscriber stations, for example, that is, thesubscriber stations 1, 2, 3, 4, and SN=5 time slots, that is, the timeslots S1, S2, S3, S4, S5. One time slot is assigned to each subscriberstation exclusively. Since in time slot S5, a node will never transmitimmediately, the nodes are able to arbitrate over time slot S5 startingwith the second bit of the time slot.

Alternatively, this seventh exemplary embodiment may also be implementedas an extension of the second exemplary embodiment. Scheduling units 15,25, 35 not only have the number SN of time slots configured, but alsothe number of time slots that are available to all subscriber stations,for example at the end of cycle C. If the time slots are firmly assignedto the subscriber stations and the time slot number is contained in thetransmitted frame, then a scheduling unit is able to detect the timeslot S5 (from the previous example). When time slot S5 has arrived, allsubscriber stations may simply arbitrate here over bus 40. Thecombination with the third exemplary embodiment is no longer necessary.

Since every one of subscriber stations 1, 2, 3, 4 is able to transmit intime slot S5, the allocation of transmit opportunity TO in time slot S5is strictly in accordance with the priority, that is, the identifier 451x of the frames, which the subscriber stations 1, 2, 3, 4 transmit intime slot S5 (strict priority scheduling). The frame identifier 451 x ofmessages 450, 460 is to be selected in this time slot S5 expediently inaccordance with the actual priority of the messages, so that the framehaving the highest priority is able to prevail.

According to an eighth exemplary embodiment, it is possible to eliminatethe arbitration phase 451 in the communication entirely or at leastpartially. As a result, in normal operation, in deterministic schedulingusing scheduling units 15, 25, 35, there is no arbitration, because oneor multiple time slots S1, S2, S3, S4 are assigned to every subscriberstation 1, 2, 3, 4. Consequently, following the assignment of the timeslots S1, S2, S3, S4 or at this operating point, the arbitration phase451 may be omitted entirely or at least partially prior to and followingthe data phase 452.

As a result, the arbitration phase 451 is now shorter than in CAN, thatis, shorter than currently defined in ISO 11898-1:2015.

In such a case, the following two operating modes exist. In a firstoperating mode, upon being switched on, all subscriber stations 1, 2, 3,4 use normal CAN frames or CAN XL frames, because arbitration may occuron the bus. As soon as all subscriber stations 1, 2, 3, 4 are inoperation and all subscriber stations 1, 2, 3, 4 are synchronized totime slots S1, S2, S3, S4, arbitration will consequently no longeroccur. For this reason, the system may now be switched into the secondoperating mode, in which the arbitration phase 451 is omitted entirelyor at least partially.

This makes it possible to avoid the case that the arbitration phase 451with its relatively long bits, which are situated before and after thedata phase 452, results in a relevant overhead in the data transmissionon bus 40 and thus in a lower net data rate.

According to a ninth exemplary embodiment, it is possible that theidentifiers 451 x, which are used for transmitting the frames 450, 460,are divided into two ranges. First, into a high priority range, which isused for those frames 450, 460 that are transmitted by a subscriberstation in the time slot assigned to the subscriber station. And second,into a low priority range, which is used for those frames that aretransmitted in a time slot that is not exclusively assigned to thetransmitting subscriber station.

In order to attempt to use a time slot that is exclusively assigned toanother subscriber station, subscriber station 1 for example, to whichfor example the currently present time slot S3 is not exclusivelyassigned, must simply use an identifier 451 x from the low priorityrange when attempting to transmit frame 450, 460. This ensures that forexample subscriber station 3, to which in this example the currentlypresent time slot S3 is exclusively assigned, would always win thearbitration and that thus this time slot S3 remains exclusively assignedto subscriber station 3. If, however, in this example at least one ofthe other subscriber stations 1, 2, 4 has something to transmit, thenthis other subscriber station 1, 2, 4 may attempt right at the beginningof time slot S3 to transmit a frame 450, 460 with an identifier 451 xfrom the low-priority range.

This has the advantage that in the present exemplary embodiment theminimum time slot duration T_S_mn remains at an arbitration bit time,whereas in the third exemplary embodiment the minimum time slot durationT_S_mn amounts to two arbitration bit times.

All the above-described developments of subscriber stations 1, 2, 3, 4,10, 20, 30 of bus system 100 and the methods carried out therein may beused individually or in all possible combinations. In particular, allfeatures of the above-described exemplary embodiments and/or of theirmodifications may be combined in any manner. Additionally oralternatively, particularly the following modifications are possible.

Although the present invention was described above with reference to theexample of the CAN bus system, the present invention may be used in anycommunication network and/or communication method. In particular, twodifferent communication phases may be used in the communication networkand/or communication method, as described above with reference to thecommunication phases 451, 452.

In particular, bus system 100 according to the exemplary embodiments maybe a communication network, in which data are serially transmittable attwo different bit rates. It is advantageous, but not an unavoidablepresupposition, that in bus system 100, at least for certain timeperiods, exclusive, collision-free access by a subscriber station 10,20, 30 to a common channel is ensured.

The number and the arrangement of the subscriber stations 10, 20, 30 inbus systems 100 of the exemplary embodiments is a matter of choice. Inparticular, subscriber station 20 in bus system 100 may be omitted. Itis possible that one or multiple subscriber stations 10 or 30 exist inbus system 100. It is possible that all subscriber stations in bussystem 100 are developed identically, that is, that only subscriberstations 10 or only subscriber stations 30 exist.

The deterministic scheduling performed by scheduling units 15, 25, 35may be switched on or switched off at any time so as to adapt to thecurrent operating states of bus system 100. In real-time operation, forexample, the deterministic scheduling is used. The deterministicscheduling may be switched off, on the other hand, in a workshop whenflashing new firmware versions.

Additionally or alternatively, it is possible that in an operation ofthe deterministic scheduling, the previously described parameters SN, W,which are used by scheduling units 15, 25, 35, may be changed as desiredin continuous operation.

1-17. (canceled)
 18. A subscriber station for a serial bus system,comprising a communication control device configured to control acommunication of the subscriber station with at least one othersubscriber station of the bus system; a transmitting/receiving deviceconfigured to transmit a transmission signal, generated by thecommunication control device, in a frame on a bus of the bus system; anda scheduling unit configured to plan a temporal access of the subscriberstation to the bus in at least one time slot of a cycle of temporallyconsecutive time slots, at least one time slot being provided in thecycle for each subscriber station of the bus for transmitting itstransmission signal and the cycle being repeated cyclically, and thescheduling unit is configured to determine, together with the othersubscriber stations of the bus in an operation of the bus system byusing a priority of the transmission signal, which time slot of thecycle the transmitting/receiving device may use for transmitting theframe for the transmission signal on the bus.
 19. The subscriber stationas recited in claim 18, wherein the scheduling unit is configured todetermine, together with the other subscriber stations of the bus, atransmission order on the bus for the subscriber stations of the bus inan at least temporarily dynamic manner and/or in an at least temporarilystatic manner.
 20. The subscriber station as recited in claim 18,wherein the scheduling unit includes a counting module configured toincrement its count value with each frame received from the bus and toincrement the count value with each transmit opportunity that haselapsed unused for a time slot, and the counting module is configured toreset the count value to a starting value when the count value is equalto a number of time slots which are provided in each cycle.
 21. Thesubscriber station as recited in claim 20, wherein the counting moduleis configured to increment its count value with every frame receivedfrom the bus following reception of a bit, which signals a beginning ofa frame, even if the received frame is later aborted by a subscriberstation transmitting the frame due to an error.
 22. The subscriberstation as recited in claim 20, wherein the scheduling unit isconfigured to approve a temporal access of the subscriber station to thebus for a next time slot of the cycle when the count value of thecounting module is equal to the number of the time slots that areprovided in the cycle, and the counting module is configured to set itscount value to 1 when the transmitting/receiving device has transmittedthe transmission signal, generated by the communication control device,in a frame on the bus or lets a transmit opportunity elapse unused. 23.The subscriber station as recited in claim 18, wherein the schedulingunit is configured to approve a temporal access of the subscriberstation to the bus in a time slot of the cycle when the subscriberstation or another subscriber station of the bus lets its transmitopportunity elapse unused.
 24. The subscriber station as recited inclaim 18, wherein the communication control device is configured topartition the frame into a first communication phase and a secondcommunication phase at least in a switch-on phase of the bus, and in thefirst communication phase, a negotiation takes place to determine whichof the subscriber stations of the bus is granted an at least temporarilyexclusive, collision-free access to the bus in a subsequent secondcommunication phase.
 25. The subscriber station as recited in claim 24,wherein a number of the time slots per cycle is greater than a number oftime slots which are assigned to the subscriber stations of the bus percycle, and in a time slot which is not assigned to any of the subscriberstations of the bus, in the first communication phase, the negotiationtakes place to determine which of the subscriber stations of the bus isgranted the at least temporarily exclusive, collision-free access to thebus in the subsequent second communication phase.
 26. The subscriberstation as recited in claim 24, wherein a minimum duration of each timeslot is a bit time of a bit of the first communication phase, and thescheduling unit is configured to transmit, in a time slot assigned tothe subscriber station, a frame having a priority that is higher than apriority of a frame which the scheduling unit is configured to transmitin a time slot that is assigned to another subscriber station of thebus.
 27. The subscriber station as recited in claim 24, wherein aminimum duration of each time slot is two bit times of a bit of thefirst communication phase, and wherein the scheduling unit is configuredto approve a temporal access of the subscriber station to the bus in asecond bit of a time slot of the cycle if another subscriber station ofthe bus lets its transmit opportunity elapse unused in a first bit ofthe time slot.
 28. The subscriber station as recited in claim 18,wherein the communication control device is configured to situate asubscriber station number assigned exclusively to the subscriber stationon the bus, in the transmission signal, and the scheduling unit isconfigured to approve a temporal access of the subscriber station to thebus in the time slot assigned to the subscriber station if thescheduling unit is able to evaluate a subscriber station number in aframe received from the bus.
 29. The subscriber station as recited inclaim 18, wherein a number of time slots which are assigned to thesubscriber station per cycle, is at least temporarily unequal to anumber of time slots, which are assigned to another subscriber stationof the bus per cycle.
 30. The subscriber station as recited in claim 18,wherein the subscriber station is configured to transmit per time slotmore than one frame on the bus, and/or a number of frames which thesubscriber station is allowed to transmit per time slot on the bus, isat least temporarily unequal to a number of frames which anothersubscriber station of the bus is allowed to transmit per time slot onthe bus.
 31. The subscriber station as recited in claim 18, wherein thecommunication control device is configured to transmit at a beginning ofa recessive bit in a time slot assigned to the subscriber station adominant pulse that is shorter than a bit time of a recessive bit if thecommunication control device lets its transmit opportunity elapseunused.
 32. The subscriber station as recited in claim 18, wherein thesubscriber station is configured in such a way that the scheduling unitmay be switched on or off depending on time requirements of thecommunication on the bus, or is configured in such a way that anoperating mode of the scheduling unit is changeable by configuration ofpredetermined parameters in a continuous operation of the bus system,the operating mode of the scheduling unit setting a predetermined modeof a communication on the bus.
 33. A bus system, comprising: a bus; andat least two subscriber stations, the at least two subscriber stationsbeing connected to one another via the bus in such a way that they areable to communicate with one another serially, wherein each subscriberstation of the subscriber stations including: a communication controldevice configured to control a communication of the subscriber stationwith at least one other subscriber station of the bus system; atransmitting/receiving device configured to transmit a transmissionsignal generated by the communication control device in a frame on thebus of the bus system; and a scheduling unit configured to plan atemporal access of the subscriber station to the bus in at least onetime slot of a cycle of temporally consecutive time slots, at least onetime slot being provided in the cycle for each subscriber station of thebus for transmitting its transmission signal and the cycle beingrepeated cyclically, and the scheduling unit is configured to determine,together with the other subscriber stations of the bus in an operationof the bus system by using a priority of the transmission signal, whichtime slot of the cycle the transmitting/receiving device may use fortransmitting the frame for the transmission signal on the bus.
 34. Amethod for communicating in a serial bus system, the method beingcarried out using a subscriber station of the bus system, the subscriberstation including a communication control device and atransmitting/receiving device, the method comprising the followingsteps: controlling, using the communication control device, acommunication of the subscriber station with at least one othersubscriber station of the bus system; and transmitting in a frame on abus of the bus system, using the transmitting/receiving device, atransmission signal generated by the communication control device, thetransmitting being in accordance with a scheduling of a scheduling unit,which schedules a temporal access of the subscriber station to the busin at least one time slot of a cycle of temporally consecutive timeslots, at least one time slot being provided in the cycle for eachsubscriber station of the bus for transmitting its transmission signaland the cycle being repeated cyclically, and the scheduling unitdetermining, together with the other subscriber stations of the bus inan operation of the bus system by using a priority of the transmissionsignal, which time slot of the cycle the transmitting/receiving devicemay use for transmitting the frame for the transmission signal on thebus.